This invention relates to a method of making an optical fiber preform for obtaining an optical fiber which is employed as a transmission line of optical communication, and more particularly to a method for the continuous fabrication of such an optical fiber preform.
The transmission line for use in optical communication is called optical fiber in this specification. This transmission line is a thread-like member usually made of transparent glass. For obtaining this, a large glass rod is prepared which is similar in sectional structure to the optical fiber, and is drawn out thin into the optical fiber. The glass rod is referred to as an optical fiber preform in this specification.
Since the optical fiber transmits an optical signal over a distance as long as several kilometers or more, sufficient care should be taken of its fabrication and quality. In an ordinary optical fiber, most of light is transmitted in the central core of the optical fiber which is high in refractive index, and a very small amount of light is transmitted in the peripheral cladding whose refractive index is lower than the core. The larger the refractive index difference between the core and the cladding is, the more the light confinement effect is produced, and the quantity of light escaping due to bending of the fiber is small. However, as the refractive index difference increases, the velocity difference also increases between the light transmitted mainly in the core and the light in the cladding, and the light pulse applied to one end of the optical fiber expands during transmission, so that high bit-rate optical pulse signal transmission is difficult. In other words, the band width of the base-band signal which can be transmitted over the optical fiber becomes narrow. In terms of physics, the light transmitted in the optical fiber is expressed by modes corresponding to electromagnetic properties, that is, the propagation constant and the electromagnetic field profile, and since the lights of the respective modes are transmitted at different velocities, expansion of the pulse width is resulted.
Two solutions to this problem are known in the art. One method is to change the structure of the optical fiber so that the lights of the respective modes are transmitted at the same velocity. An concrete structure thereof is disclosed in detail in U.S. Pat. No. 3,614,197 issued to Nishizawa et al and "An Optical Waveguide with the Optimum Distribution of the Refractive Index with Reference to Waveform Distribution" by Shojiro Kawakami and Junichi Nishizawa, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT16, No. 10, pp. 814-818, October 1968. That is, the refractive index profile in the section of the optical fiber is selected so that the refractive index decreases from the center of the optical fiber in the radial direction substantially followed the function, as given by the following equation: EQU n.sub.(r) = n.sub.0.sup.2 [1 - (r/R).sup.2 + .delta.(r/R).sup.4 + . . . ]
where .delta. is a constant from 2/3 to 1, n.sub.0 the refractive index of the center of the optical fiber and R the radius of the optical fiber.
With the transmission mode of the optical fiber having such a refractive index profile, since the transmission velocities of the respective modes are substantially the same, expansion of the light pulse is very little. The optical fiber of such a structure is called graded index type optical fiber.
The other method is to reduce the velocity dispersion by decreasing the number of transmission modes. The simplest form of this structure is that transmission takes place only in a single mode.
The requirements for transmission only in a single mode are set forth in "Fiber Optics-Principles and Applications" by N. S. Kapany, Academic Press (1967). The optical fiber is needed to have the structure satisfying the following conditions: EQU R = 2.pi.a/.lambda. .sqroot.n.sub.1.sup.2 - n.sub.2.sup.2
where a is the radius of the core, the wavelength of light transmitted, n.sub.1 the refractive index of the core, and n.sub.2 the refractive index of the cladding. Where the value of R is smaller than 2.405, only one mode is transmitted, so that pulse broadening is very little. The optical fiber produced by this method has an abrupt refractive index change at the boundary between the central portion of high refractive index and the cladding portion of lower refractive index, and this type of optical fiber is referred to as the step index type optical fiber.
As described above, whether the graded index type or step index type, the optical fiber is required to have the structure of the desired type. To this end, it is necessary to produce a optical fiber preform which exactly complies with the structural requirements of the optical fiber desired to obtain.
Conventional methods for the manufacture of optical fiber preforms are set forth in U.S. Pat. No. 3,737,292 issued to D. B, Keck et al and U.S. Pat. No. 3,823,995 issued to L. L. Carpenter et al. With the prior art methods, a substantially cylindrical starting member is prepared and glass fine particles produced as by flame-hydrolysis of glass raw materials are deposited in layers on the entire area of the peripheral surface of the starting member. Then, the deposited layers of the glass fine particles are vitrified in a high temperature furnace, after which the starting member is removed, and only the deposited layers of the glass to form a preform having a solid cross-sectional area are by heating to collapse the hollow portion. In this case, if the components of the abovesaid layers are each selected to be a little different from the component of the immediately inside layer, the graded index type optical fiber preform can be obtained. On the other hand, where a plurality of layers of a constant thickness for the core of high refractive index are deposited on the starting member and a plurality of layers for the cladding of lower refractive index are deposited on the core, the step index type optical fiber preform can be obtained. In other words, the layers are deposited on the starting member one by one, with the concentration of a dopant for controlling the refractive index of each layer changed.
The above method has a defect that such a porous deposit of glass fine particles formed on a solid starting member is likely to be cracked as by small temperature variations because of the thermal expansion coefficient difference and density difference between the starting member and the deposit. With this method, however, fabrication of large optical fiber preforms is easier than the other conventional methods such for example, as set forth in U.S. Pat. No. 3,737,293 issued to R. D. Maurer.
With the method of R. D. Maurer, a quartz tube is prepared, a plurality of glass layers of low refractive index are deposited on the interior surface of the quartz tube and then a plurality of glass layers of high refractive index are deposited on the low refractive index layers and finally the composit structure are collapse to achieve a solid cross-sectional structure to the center. This method has a defect of difficulty in the fabrication of large optical fiber preforms.
One object of this invention is to provide a continuous optical fiber preform fabrication method with which it is possible to obtain an optical fiber preform of desired characteristics easily and continuously, even if it is large and long.
Another object of this invention is to provide a continuous optical fiber preform fabrication method which is high in productivity.
Another object of this invention is to provide a continuous optical fiber preform fabrication method with which it is possible to obtain an excellent optical fiber preform which is formed of a deposit of glass fine particles to the core and which is free from alient substances such as a starting material and so on, stable thermally and mechanically and difficult of cracking.
Still another object of this invention is to provide a continuous optical fiber preform fabrication method which does not require removal of a starting member and is simple in manufacturing steps.